Shear Bond Strength 1

Shear Bond Strength of Orthodontic Brackets

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Shear Bond Strength 2

Table of Contents

Introduction .......................................................................................................................................3

Types of Orthodontic brackets .........................................................................................................3

Orthodontic bracket failure .............................................................................................................3

Factors determining the effectiveness of orthodontic brackets..........................................................4

Vector Forces Acting on orthodontic bracket ....................................................................................4

Literature Review ...............................................................................................................................6

Method............................................................................................................................................ 12

Bibliography ..................................................................................................................................... 14

Shear Bond Strength 3

Introduction The procedure of direct bonding of orthodontic brackets to the enamel of the teeth has become a

routine clinical procedure in dental settings (Brantley & Eliades, 2011). The direct bonding of

brackets has provided an esthetic alternate to the former orthodontic bonding. The relatively

modern procedure of binding brackets to the surface of the teeth enamels requires considerable

expertise as there are various factors that need to be considered prior to the procedure. These

factors include enamel conditioning agents, base design, cement luting agents and bracket

material. The most important factor to consider is that the shear bond strength of these

orthodontic brackets need not only have sufficient strength to resist fracture overtime, but to

have enough strength to evade bond failure (Moin & Dogon, 1978). Orthodontic brackets that

break easily may cause damage to the enamels of the teeth and leading to inconvenience to the

orthodontist and the patient, as well as increased financial cost to the orthodontist, and ultimately

to the patient (Jassem, et al., 1981). While incidences of bond fracture, failure or debonding are

not uncommon during the course of the treatment, most orthodontists and patients see this as an

undesired happening in the sequences of events. Therefore, it can be established that it is

necessary to understand the device of orthodontic brackets, as well as to understand the internal

mechanics and the role of a variety of products and material available to be used in the procedure

in order to understand, and ultimately avoid the failure of the orthodontic bond and ensure the

success of this clinical endure.

Types of Orthodontic brackets The orthodontic brackets that are currently at the orthodontist’s disposal are extensive and varied

(Brantley & Eliades, 2011). The brackets may be made of ceramic, metal, plastic or a

combination of these materials. An orthodontist may use 2 categories of cement to attach bracket

to his patient’s enamel, the enamel may be conditioned or non-conditioned. The 2 categories are;

resin composite or resin reinforced glass ionomers. Resin composites require conditioning of the

enamel surface; this cement requires dry bonding enviir0mment, whereas resin reinforced glass

ionomers can bond in non-conditioned or wet environments (Jr, 1983).

Orthodontic bracket failure The debonding of the orthodontic brackets from the surface of the enamel may he labelled as

bond failure (Titley, et al., 2003). Bond failure may result in iatrogenic damage to the surface of

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the enamel or traumatic injury to the surrounding structures. Broadly speaking, the sites of bond

failure can be divided into three anatomical sites, which are; within bracket, between cement and

enamel surface and between cement and bracket (Jassem, et al., 1981) (H, et al., 1998). The

fracture of a bonded orthodontic bracket is not an uncommon occurrence. An orthodontic bracket

must resist a force of 6-8 MPa in order to avoid breaking and maintaining clinical success.

However, a displacement force of over 10 MPa can lead to bond failure (Brantley & Eliades,

2011).

An Index has been developed to quantify the amount of cement that remains on the enamel

surface after a bond failure (Brantley & Eliades, 2011). The ARI or adhesive remnant index is

dependent on the type of bracket material and type of cement used; however, there is no

evidence of a co-relationship between the two variables (Alexandre, et al., 1981) (Jassem, et al.,

1981).

Factors determining the effectiveness of orthodontic brackets The effectiveness of orthodontic brackets are determined by two important factors, which are;

the durability of the attachment of the bracket to the surface enamel and the proper determination

of the corrective force vectors. The latter factor, which is the force vector is generated by the

elastic component of the corrective device and should be treated as the resultant of tension, shear

and torsional forces, which, depending on the location of the bracket may have different values.

The former factor, which is durability of the bracket-cement-enamel interface is calculated in

torque and is not a homogenous parameter (Johnson, et al., 1976) (Jassem, et al., 1981).

When assessing the cause of bond failure directional characteristics are taken into account by a

semi-quantitative assessment using ARI or the adhesive remnant index (Siomka & Powers,

1985).

Vector Forces Acting on orthodontic bracket In solid mechanics as in orthodontic brackets, torsion is the twisting of an object due to an

applied torque, this torque is one of the forces acting on the bracket-cement-enamel interface. It

is expressed in newton metres (N·m) or foot-pound force (ft·lbf). In units perpendicular to the

torque axis, the resultant shear stress in this unit is perpendicular to the radius of the contact

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surface ( figure 1 a) (Brantley & Eliades, 2011). The red arrow in the figure 1 a indicates to the

direction of the force.

Tensile force is a measure of the ability of a material, in this case of bracket or bon to withstand

a longitudinal stress, communicated as the greatest stress that the material can stand without

breaking or enduring damage (figure 1 c) (Goyal & Goyal, 2011).

Shearing forces are unaligned forces pushing one part of the corrective device in one direction,

and another part of the body in the opposite direction (Jassem, et al., 1981). When the forces are

aligned into each other, they are called compression forces (figure 1 b).

The red arrows in the figure 1 indicate to the forces acting on the teeth. The vector of the

combined forces dictates the ultimate force acting on the bracket which needs to be overcome to

maintain an effective corrective device and thereby avoiding bond failure (Jassem, et al., 1981).

Figure 1

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Literature Review Obtaining adequate force during orthodontic treatment will certainly result in an optimal tissue

response and satisfactory tooth movement (Brantley & Eliades, 2011).

The types of dental brackets available to the orthodontists are extensive and varied; however, the

two basic types are metal or ceramic (H, et al., 1998) (Ødegaard & Segner, 1988). The choice of

material of the bracket is dependent on a variety of factors, such as; severity of presented

pathology, the aim of the procedure, the extent of the treatment, preference of the orthodontist,

patient’s financial obligations or choices and various others. Ultimately, both materials have its

advantages and disadvantages (John Gwinnett, 1988).

One of the most common reasons for popularity of ceramic braces is their aesthetic appeal, in

comparison to the metal counterpart. The shear bond strength of ceramic bracket is superior to

that of metal brackets in a variety of adhesives used. However, it has been established that the

ceramic bracket occurred predominantly in the enamel-adhesive interface, while the site for bond

failure for the metal bracket was found to be mainly in the bracket-adhesive interface. It has been

determined that the shear bond strength between ceramic bracket and adhesive is stronger than

that of the shear bond strength between the enamel surface and the adhesive. It has been

suggested that ceramic brackets offer an effective alternative to their metal counterpart. It can be

determined that ceramic brackets combine esthetics with bond strength that is analogous to and

as dependable as their metal counterparts (John Gwinnett, 1988) (Jassem, et al., 1981).

In order to achieve optimal treatment, clinically adequate bond strengths to enamel for metal

orthodontic brackets should be sufficient to withstand normal orthodontic forces and masticatory

loads, in addition to being aesthetic and easily removed at the end of treatment, without

damaging the enamel surface (Brantley & Eliades, 2011). The underlying cause of bond failure is

calculated by semi-qualitative analysis by using the adhesive remnant index or ARI.

Several factors influence the bond strength of brackets (Maijer & Smith, 1981)including a wide

range of the available etching agents’ and adhesives, the size and design of the bracket base,

masticatory forces and temperature, among others (Maijer & Smith, 1981).

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The most commonly used etching agent is phosphoric acid. Researcher (Retief, 1975) suggests

the use of 50% phosphoric acid solution as a conditioning agent, but the optimal phosphoric acid

concentration should be determined for each adhesive system.

Other factor that deeply influences sheer bond strength is the bracket/adhesive system (Brantley

& Eliades, 2011)an adhesive must be able to deal with numerous deleterious conditions in the

oral cavity, such as constant moisture and the considerable masticatory stress as well as applied

orthodontic stress.

In a study titled, ‘Shear Bond Strengths of orthodontic brackets cemented to bovine enamel with

composite and resin modified glass ionomer cements’ authors (Titley, et al., 2003) studied the

effects of short and long term storage of the shear bond strength of metal, ceramic and

polycarbonate orthodontic brackets bases using resin-modified glass ionomer cements and resin

composite. The authors of the study concluded that the bracket base and cement combination

produced sustainable combination in all cases and that devices are durable overtime, however,

the authors warn that the selection of cement is very important in patients who are at high risk for

caries (Titley, et al., 2003).

Authors’ (Rastelli, et al., 2010) conducted a highly regulated study to evaluate the shear bond

strength of stainless steel bracket with fluoride releasing composite resins compared to other

adhesive mediums. The authors concluded that while all materials tested in this investigation

have adequate SBS to meet clinical and/or corrective needs, Concise showed greater resistance

than Rely-a-Bond and Ultra bond. The authors also found that the adhesive remnant index was

similar between all groups and although bon failure did occur, there was no damage to the

enamel surface, except in case of Concise, which exhibited enamel fractures.

The size and design of a bracket base can also affect bond strength (H, et al., 1998). Brackets

with a circular concave base design produced greater bond strength than the brackets with mesh

bases. It has been determined that the larger the mesh spacing, the greater the bond strength of

the corrective device ( (Wang, et al., 2004).

Due to the higher masticatory forces generated in the posterior regions of the mouth or by the

differences in enamel micro morphology (Knoll, et al., 1986), there is a clinically higher failure

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rate among bonded brackets on posterior teeth than on anterior teeth (Garlic 1977, Zachrisson

1977). Authors (Knoll, et al., 1986) also suggests that the no uniformity of the resin thickness

between the enamel and bracket base for posterior teeth may account for the observed

differences.

Temperature fluctuations in the oral environment is an important factor that needs to be

considered. Because of the temperature fluctuations that occur in the oral environment, the

effects of temperature cycling on the bond strengths of bonding resins to etched enamel have

been evaluated (Maijer & Smith, 1981). Temperature cycling did not have a significant effect;

however, on shear bond and rebond strengths when compared to the shear bond and rebond

strengths of uncycled specimens of a low-viscosity bonding resin to etched enamel (Jassem, et

al., 1981).

The reuse of orthodontic brackets and the consequent rebonding procedures are becoming

increasingly popular because they minimize waste and cost to the orthodontist and ultimately to

the patient. Bond strength after one reconditioning cycle ranged from 45% to 75% of initial bond

strength for different types of adhesives (Wright & Powers, 1985). This finding supports another

study where the bond shear strength values after reconditioning were from 65% to 84% of initial

shear strength values, depending on the bracket brand (Mascia & Chen, 1982).

Since there are varieties of bracket bases that are available to the orthodontists for corrective

procedure under discussion, there have been various studies evaluating the success and

advantage of one bracket base to another. Authors in a study evaluated bracket bases from 7

different manufactures using a SEM or scanning electron microscope (Maijer & Smith, 1981).

The bracket bases were bonded to human premolars with Dynabond and stored in water for

twenty four hours before being tested for shear failure. The study concluded that the best resin

penetration and bond strength were obtained with a fine mesh bracket base of the woven mesh

type. It also determined that the bracket bases should be designed to prevent air entrapment

under the base, which may precipitate bond failure. The authors concluded that the weld spots on

attachment bases should be avoided to prevent poor seal and cause complications, since weld

spots reduce retentive area the weld spurs could be responsible for lower bond strengths in some

samples of mesh and foil (Maijer & Smith, 1981).

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Another factor that determines the strength and thereby the durability of orthodontic brackets is

the storage medium that may be used to store these brackets. There are a variety of mediums that

can be used in order to prolong the life of the brackets and avoid microbial infestation (Brantley

& Eliades, 2011). A variety of in vitro studies have been conducted to evaluation the effects of

storage media on the SBS or shear bond strength of orthodontic brackets. Storing teeth in media

other than water may decrease fungal, bacterial and viral growth, which in turn may prevent

increased financial cost and inconvenience (Sachdeva, et al., 2012). The storing may also prevent

enamel desiccation prior to the testing or any other purpose the teeth are being stored for. A

comparison of six storage medium, which are; distilled water, 10% formalin, saline solution of

0.9% sodium chloride, 70% ethanol, 3% hydrogen peroxide and artificial saliva. It was

determined that the formalin had the highest mean shear bond strength, while ethanol had the

lowest mean shear bond strength. The shear bond strength of isotonic saline solution and distilled

water were about 7.59 and 6.15 MPa respectively, which was comparable to the clinically

acceptable shear bond strength of 6-8 MPa. The study recommended saline solution and distilled

water as the most effective storage media for the orthodontic bracket storage (Sachdeva, et al.,

2012).

The ultimate shear bond strength of an orthodontic bracket is determined by the interplay of

various factors, amongst which, one is the media used to prepare the enamel for bonding to the

brackets i.e. surface preparation of the enamel. A comparison of enamel prepared by YAG laser

with two different powers to the conventional acid etching was carried out using 1 W and 1,5 W

of YAG laser power and 37% phosphoric acid (Hosseini, et al., 2012) . The study subjected all

the test subjects to thermo cycling process and used the adhesive remnant index to evaluate the

different etching types. The study concluded that the using YAG laser was a favorable alternate

to using conventional acid etching for surface conditioning. The authors of the study determined

that while the shear bond strength obtained at both powers of the YAG laser is similar to that of

conventional etching with 37% phosphoric acid, the high variability of values in bond strength of

YAG surface conditioning should make it a favorable alternate (Hosseini, et al., 2012).

With the aim of achieving a strong and reliable bonding between the bracket and the tooth, share,

tensile and torsion tests have been widely performed (Brantley & Eliades, 2011). Using of shear

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loading has been very popular due to the relative simplicity of the experimental configuration

and the presumably increased reliability of similarity to debonding that occurs during treatment.

By evaluating different adhesives in metal and ceramic brackets it has been shown that the shear

bond strength of ceramic brackets is superior to that of metal brackets (Reddy et al. 2003).

The tensile test is less commonly performed. Al-Munajed et al. (2000) have evaluated the

performance of a cyanoacrylate orthodontic adhesive with regard to tensile bond strength in

comparison with a conventional no-mix orthodontic composite adhesive using stainless steel and

ceramic brackets, suggesting that cyanoacrylate orthodontic adhesive is unsuitable as a bonding

agent in either case (H, et al., 1998).

Merrill et al. (1994) have evaluated the torsional forces that were best suited for debonding

ceramic brackets. It was found that the torsional bond strength of chemically retained brackets

was significantly higher than mechanically retained brackets. Using different adhesives, Kao et

al. (1995) have investigated the torsional bond strength of ceramic brackets bonded to human

enamel. They concluded that dedonding or bond failure of ceramic brackets under a steady

torsional load caused no substrate surface alterations regardless of the adhesive used, which

means that there is minimal to no damage to the enamel surface, making it one of the better

choice of bracket base material.

Recently, many clinicians have shifted to one-step self-etch adhesive systems, also referred to as

all-in-one adhesives, in which manufacturers have incorporated all of the primary components of

adhesive systems (etchant, primer, and bonding resin) into a single solution (Catalbas, et al.,

2011). All-in-one adhesives are user friendly, because fewer steps are required for bonding

(Hegde and Manjunath 2011) (J & Bergland, 1984).

A study conducted by authors (Pawlus, et al., 2013) evaluated the durability of the bond between

orthodontic and enamel via tensomeric and planimetric evaluation. The objective of this study

was to evaluate the real strength of adhesives used in the corrective procedure and to introduce a

copyrighted device, generating multiple vectors of strength between dental brackets and the

surface enamel. The study aimed to evaluate the three basic forces acting against the orthodontic

bracket, which are; torsional force, tensile force and shearing force. The study found that the

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resistance was highest for torsional stress, weaker for shear stress and weakest for tension stress

(Pawlus, et al., 2013). The authors concluded that effective strength to oppose the three forces is

necessary in order to ensure the clinical success of the corrective device.

One of such adhesives is the XP BOND dental adhesive system. It has been shown that he

bonding potential of XP BOND used with the activator or light cured in combination with self-

or dual-curing mode outperformed that of a control adhesive-cement system (Raffaelli et al.

2007). In addition, when comparing four commercially available adhesive systems (two total-

etch and two all-in-one), it was shown that the XP BOND showed the highest bond strength for

both the moist and dry dentin conditions (Hedge and Manjunath 2011). Similarly, when

comparing the micro tensile bond strength of three different totals etch adhesives; XP Bond

showed the greatest values of micro tensile bond strength under both conditions. Moist substrate

increases the values of micro tensile bond strength for the adhesives tested. It can be determined

that XP BOND is a superior adhesive system as compared to other three systems that were the

object of this study (Orellana et al. 2009).

As adhesive systems improve, the bases became smaller and smaller other variables have

become more important to the over-all bond strength of bracket bases (SK, 1999). A balance

between normal dental forces and overall bracket bond strength must be achieved to improve

patient’s treatment. Failure to do so can result in bond failure and fracture, which in turn can lead

to iatrogenic damage to the enamel of the teeth. While, bond failure is not an uncommon

occurrence during the course of the corrective procedure, it has been determined that such

mishap can lead to increased cost, longer duration of treatment and psychological distress to the

patient.

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Method The study consisted of 180 human teeth. The author of study ensured that the teeth was devoid of

any defects, such as; infection, caries, breaks or decay. The experiment consisted of 60 incisors,

60 premolars and 60 molars. The teeth were stored in normal saline (0.9% NaCl) in a closed

plastic box. Teeth were divided into 6 group each one (10 incisors, 10 premolars and 10 molars) .

A pilot study done on typodont jaws, as shown in figure 2, with acrylic teeth were used with

APC bracket and UR2 and LR1 displaced 2mm. A 014 Nickel titanium wire was used for the

purpose of this study. The electronic calibration is used to deflect the wire to the slot of bracket

of displaced tooth and the force recorded. The test is repeated done when the above teeth

displaced 4mm with same wire. In similar way the test done using 016 nickel titanium wire and

the force required registered to estimate the starting point of the horizontal force used in the

experimental.

Each group was tested at a separate time. On the day of the test, teeth were placed into acrylic

block (figure 4) dimensional approximately height 30 mm , width 15mm and depth 15 mm with

facial surface of the teeth expose and parallel to the chisel of the introns machine (Figure 3) .

Following the acrylic sit, the surface of teeth etched with phosphoric acid 37% for 10 – 15 sec.

The teeth are subsequently washed and dried. Then, the XB bond resin is applied followed by

light curing for 10- 20 sec, each tooth bond with corresponding bracket APC (MBT prescription)

which has preload composite.

Figure 2 Figure 3

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Figure 4

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